Government agencies and industry standard setting groups are reducing the amount of allowed emissions in stoichiometric and diesel engines in an effort to reduce pollutants in the environment. For example, over the past decade, increasingly more stringent heavy duty on-highway engine emission regulations have led to the development of engines in which NOx and diesel particulate emissions have been reduced by as much as seventy percent and ninety percent, respectively. Proposed regulations for new heavy duty engines require additional NOx and diesel particulate emission reductions of over seventy percent from existing emission limits. These emission reductions represent a continuing challenge to engine design due to the NOx-diesel particulate emission and fuel economy tradeoffs associated with most emission reduction strategies. Emission reductions are also desired for the on and off-highway in-use fleets.
Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines. These forms are diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties that are vastly different from each other. The diesel engine controls the start of combustion (SOC) by the timing of fuel injection. A spark ignited engine controls the SOC by the spark timing. As a result, there are important differences in the advantages and disadvantages of diesel and spark-ignited engines. The major advantage that a spark-ignited natural gas, or gasoline, engine (such as passenger car gasoline engines and lean burn natural gas engines) has over a diesel engine is the ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines is higher thermal efficiency.
One reason for the higher efficiency of diesel engines is the ability to use higher compression ratios than spark ignited engines because the compression ratio in spark ignited engines has to be kept relatively low to avoid knock. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels that are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion a large fraction of the fuel exists at a very fuel rich equivalence ratio, which is known to lead to particulate emissions. Spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures that tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. A second consideration is that the combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Lean burn spark ignited engines, on the other hand, burn their fuel at much leaner equivalence ratios which results in significantly lower temperatures leading to much lower NOx emissions. Stoichiometric spark ignited engines, on the other hand, have high NOx emissions due to the high flame temperatures resulting from stoichiometric combustion. However, the virtually oxygen free exhaust allows the NOx emissions to be reduced to very low levels with a three-way catalyst.
Recently, some members of industry have directed their efforts to another type of engine that utilizes homogeneous charge compression ignition (HCCI) to reduce emissions. Engines operating on HCCI principles rely on autoignition of a premixed fuel/air mixture to initiate combustion. The fuel and air are mixed, in the intake port or the cylinder, before ignition occurs. The extent of the mixture may be varied depending on the combustion characteristics desired. Some engines are designed and/or operated to ensure the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Additionally, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. HCCI combustion is characterized in that the vast majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge by the time of ignition and throughout combustion and combustion is initiated by compression ignition. Unlike a diesel engine, the timing of the fuel delivery, for example the timing of injection, in a HCCI engine does not strongly affect the timing of ignition. The early delivery of fuel in a HCCI engine results in a premixed charge that is very well mixed, and preferably nearly homogeneous, thus reducing emissions, unlike the stratified charge combustion of a diesel, which generates higher emissions. Preferably, HCCI combustion is characterized in that most of the mixture is significantly leaner than stoichiometric to reduce emissions, which is unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion
Other members of industry have moved to “dual mode” engines that operate on both a gaseous fuel mixture and diesel fuel. These engines operate in HCCI mode at part load and in diesel mode or SI mode at full load. As a result, dual mode engines produce low emissions similar to spark ignited natural gas engines and high thermal efficiency similar to diesel engines. In particular, in known dual mode engines using diesel fuel and natural gas at high load, only a small amount of diesel fuel is required to start ignition and the emissions produced would be similar to a spark ignited natural gas engine. Under other conditions when substantial diesel fuel is injected, the emissions produced would be similar to a conventional diesel engine.
In order to monitor emissions, it is required to detect engine combustion conditions during engine operation. Of all the measuring methods for detecting engine combustion conditions, ion current measurement has been considered to be highly useful because it can be used for directly observing the chemical reaction resulting from the engine combustion. However, ion current detectors are typically incorporated into glow plugs. For example, an electric conductive layer made of platinum is formed on a surface of the heating element of the glow plug and is electrically insulated from the combustion chamber and the glow plug clamping fixture.
In these glow plugs, ignition and combustion of fuel are generally promoted by a heating action of the glow plug heating element when the engine starts at low temperature. The heating state of the heating element usually continues after warm-up of the engine has been completed until the combustion is stabilized (generally, referred to as “afterglow”). After completion of the afterglow, the heating action of the glow plug is stopped and the process of detecting ion current is started. Carbon adheres to the circumference of the ceramic heating portion of the glow plug and reduces the insulation resistance between the exposed electrode used for ion current detection and the grounded portion (plug housing and cylinder head) that is insulated from the electrode. In this case, a flow of leakage current may be created through the adhered carbon even if no ion is derived from the combustion gases. When this happens, the ion current detected shows a waveform different from a desired one due to occurrence of the leakage current, and such an incorrect detection result causes deterioration in the accuracy of ignition stage and flame failure detections. Furthermore, the electrode is almost completely exposed into the combustion chamber and the space between the housing and the electrode is narrow. For this reason, there is a danger that the electrode is shorted to the ground and the housing is made conductive due to adhesion of carbon to the electrode surface, resulting in an error in detecting ion current.
Additionally, since the ion current detecting electrode supported at the tip of the glow plug directly touches a flame having a high temperature, stresses tend to be concentrated in the neighborhood of the ion current detecting electrode and could damage the ceramic glow plug such as to crack it.